There are various different types of engines that use more than one fuel. One type is known as a direct injection gas (DIG) engine, in which a gaseous fuel, such as liquefied natural gas (LNG), is injected into the cylinder at high pressure while combustion in the cylinder from a diesel pilot is already underway. DIG engines operate on the gaseous fuel, and the diesel pilot provides ignition of the gaseous fuel. Another type of engine that uses more than one fuel is typically referred to as a dual-fuel engine, which uses a low-pressure gaseous fuel such as natural gas that is mixed at relatively low pressure with intake air admitted into the engine cylinders. Dual-fuel engines are typically configured to operate with liquid fuel such as diesel or gasoline at full power. The gaseous fuel is provided to displace a quantity of liquid fuel during steady state operation. The air/gaseous fuel mixture that is provided to the cylinder under certain operating conditions is compressed and then ignited using a spark, similar to gasoline engines, or using a compression ignition fuel, such as diesel, which is injected into the air/gaseous fuel mixture present in the cylinder.
In dual fuel engines, the gaseous fuel is stored in a pressurized state in a pressure tank, from which it exits in a gaseous state before being provided to the engine. In DIG engines, however, the gaseous fuel is stored in a liquid state at low pressure, such as atmospheric pressure, and at low, cryogenic temperatures in a liquid storage tank. When exiting the liquid storage tank, the liquefied gaseous fuel requires heating to ultimately evaporate and reach a gaseous state before or when it is provided to the engine cylinders.
Pumping liquids typically requires maintaining a net suction pressure above the vapor pressure of the liquid being pumped at a given temperature. This is called the net positive suction head (NPSH) requirement. Whenever the liquid pressure drops below the vapor pressure, liquid boiling occurs, and cavitation and loss of prime of the pump may result. Vapor bubbles may reduce or stop the liquid flow, reduce pump efficiency, lower the mass of the compressed liquid, and possibly damage the system.
Cryogenic liquids, such as liquefied natural gas (LNG), are stored at saturated conditions, and so typically the only available NPSH is the static head caused by the level of the liquid in the storage vessel. This NPSH may be very low, particularly when the storage tank is nearly empty.
U.S. Pat. No. 7,410,348 is entitled, “Multi-Speed Compressor/Pump Apparatus.” The '348 patent is directed to a pumping/compressing apparatus that includes a motor and a two-stage device including at least one reciprocating piston, an inlet stage chamber, an outlet stage chamber, an inlet, and an outlet. The piston is arranged to be reciprocated at multiple speeds for either pumping or compressing operations. When the piston is reciprocated at a first speed, the device pumps liquid to the outlet. When the piston is reciprocated at the second speed, which is higher than the first speed, the device compresses gas and provides it to the outlet.
It will be appreciated that this background description has been created by the inventors to aid the reader, and is not to be taken as an indication that any of the indicated problems were themselves appreciated in the art. While the described principles can, in some aspects and embodiments, alleviate the problems inherent in other systems, it will be appreciated that the scope of the protected innovation is defined by the attached claims, and not by the ability of any disclosed feature to solve any specific problem noted herein.